Compare (rate)
Traffic Stop Light Signals
using Braking g's

Page Outline

• Introduction
• Standards
• Variations
• Deceleration
• Calculate Braking g's
  -- Braking g's Calculator
• Examples

Some traffic signals require harder braking than others. Braking depends on vehicle speed, yellow light duration, stop and clear points, and driver reaction time. A way to compare different traffic signals (including controversial red light cameras*) is to determine maximum expected braking action required. A convenient way to measure braking action is to equate (compare) vehicle deceleration to the gravity acceleration constant (g).

* -- RED LIGHT CAMERAs that automatically photograph and/or video record vehicles alleged to have run a red light are controversial for several reasons that include;

• cameras do not improve safety or fix problematic intersections
  -- dangerous intersections can usually be fixed by increasing yellow light duration,
  -- longer yellow light duration has lower braking g's (safer).
• the process is automatic
  -- a police officer does not have to see alleged violation,
  -- alleged violator cannot cross examine electronic circuits.
• cameras have been known to cause collisions
  -- some drivers aware of a red light camera brake too soon and get rear-ended.
• camera's real purpose is to collect lots of $money$
  -- up to half of fine can go to a 3rd party (private company that maintains system).

The Manual on Uniform Traffic Control Devices (MUTCD), established by federal law in 1966, sets national standards for every sign, signal, pavement marking, and traffic control signal device in the USA. The standard (published by the Federal Highway Administration -- FHwA) is intended to insure signs and signals LOOK and are USED in the same manner everywhere in the USA.

The MUTCD only provides guidance for vehicle position when a traffic signal changes to red (no formal standards). Basically each State sets the law regarding yellow light duration and vehicle position, and some of these laws are open to interpretation. Many State laws use language taken directly and/or modified from the Uniform Vehicle Code (UVC). The UVC code is consistent with the MUTCD concerning yellow light duration and vehicle position when light changes red. The MUTCD recommends yellow light duration be 3 to 6 seconds (longer times for greater speeds); and on/after red vehicle cannot enter intersection.

Different states and countries have different standards regarding vehicle location when a traffic light signal changes to red. Variations for vehicle location when signal changes from yellow to red include but not limited to;

• Vehicle cannot enter intersection (MUTCD standard).
• Front of vehicle must be clear of intersection.
• Entire vehicle must be clear of intersection .
• Vehicle cannot enter intersection on yellow (few states).

If the entire vehicle (not just front bumper) must be clear of the intersection (or some other point) when light changes red, braking (deceleration) will also depend on vehicle length -- the greater the vehicle length the harder the required braking.

Acceleration (a) is the change in speed over the change in time, and is positive for increasing speed and negative for decreasing speed (deceleration). At a traffic stop light deceleration is a function of;

Driver reaction time includes driver recognizing light has changed, deciding whether to continue or brake, and applying action (brake if necessary -- remove foot from gas pedal and apply brake). Reaction times vary greatly with situation and from person to person between about 0.7 to 2.5 seconds or more. Some accident reconstruction specialists use 1.5 seconds as average driver reaction brake time. However, a controlled study by the University of Iowa published in 2000 (IEA2000_ABS51.pdf) found average driver reaction brake time to be 2.3 seconds. Some state (such as California) DOT's (Department of Transportation) use a driver reaction time of 2.5 seconds for calculations.

Driver reaction time;
0.7 seconds -- about as fast as it can get,
1.5 seconds -- very good (fast) reaction time,
2.3 seconds -- average,
2.5 seconds -- slightly longer than average.

Figure 1 --
Deceleration Model

Four numbers required to compute Vehicle Deceleration (-a); vo = vehicle speed. ty = yellow light duration. tr = driver reaction time. di = distance from stop line to point vehicle must be at/past on red.

If entire vehicle (not just front bumper) must be clear of some point (intersection, etc.), required deceleration will depend on vehicle length -- the longer the vehicle the greater the required deceleration. The distance (d_i) between stop point and point vehicle must be at or past on red is greater for longer vehicles, increasing requried deceleration.

Deceleration (-a) can be equated (compared) to the acceleration due to gravity (g's) constant. By International definition 1 g is a change in speed of 9.80665 meters per second every second (g = 9.80665 m/s²), or about 32.2 ft/s².

The longer the yellow light duration (t_y), the lower the braking g's (G_b). The greater vehicle speed (v_o) and/or the greater driver reaction time (t_r) and/or the greater the distance between stop point and point vehicle must be at or past when light changes red (d_i), the greater the braking g's (G_b).

Under ideal conditions (dry closed track and factory fresh tires) it has been claimed a professional driver can brake at a rate of 1 g or more. About 0.8 g's is the hardest braking (approximately at or near a dead skid) most people will (can) experience in a street vehicle. Adaptive Cruise Control (ACC) systems (vehicle radars) automatically brake when approaching other vehicles -- maximum braking varies from 3 to 5 m/s², or about 0.3 to 0.5 g's.

The following two graphs plot braking g's (G_b) versus yellow light duration d_y when d_i = 0 for speeds v_o between 20 and 65 mph (reaction time fixed). The only difference between the two graphs is driver reaction time. The first graph uses a reaction time of 1.5 seconds, the second graph uses a reaction time of 2.3 seconds.

If distance between stop point and point vehicle must be at or past is 0, vehicle speed 35 mph, yellow light duration 3.5 seconds, and driver reaction time 1.5 seconds (d_i = 0, v_o = 35 mph, t_y = 3.5 sec, t_r = 1.5 sec), then G_b = 0.4 g's (moderate braking).

If distance between stop point and point vehicle must be at or past is 0, vehicle speed 35 mph, yellow light duration 3.5 seconds, and driver reaction time 2.3 seconds (d_i = 0, v_o = 35 mph, t_y = 3.5 sec, t_r = 2.3 sec), then G_b = 0.66 g's (hard braking).

The following table shows braking g's (G_b) when driver reaction time (t_r) is 2.5 seconds and stop line and clear point the same (d_i = 0) given yellow light duration (half second increments) and speed (5 mph increments).

If distance between stop point and point vehicle must be at or past is 0, vehicle speed 35 mph, yellow light duration 3.5 seconds, and driver reaction time 2.5 seconds (d_i = 0, v_o = 35 mph, t_y = 3.5 sec, t_r = 2.5 sec), then G_b = 0.80 g's (border line extreme braking).